Radio frequency receiver



y H. A. WHEELER 1,960,984

RADIO FREQUENCY RECEIVER Filed April 5, 1933 RADIO RECf/ YER 19f!PONY/K. T0 1. E

[9 AVC INVENTOR HAROLD A.WHEELER ATTORNEYS Patented May 29, 1934 PATENTOFFICE 7 1,960,984 namo FREQUENCY RECEIVER Harold A; Wheeler, GreatNeck, N. Y., assignor to Hazeltine Corporation, a corporation ofDelaware Application April 5, 1933, Serial No. 664,480

10 Claims. (Cl. 250-20) The present invention relates to the selectionof a radio-frequency signal being transmitted on one carrier frequencyand the simultaneous reduction of interference being transmitted onother frequencies not greatly .different from the frequency of thedesired signal, More particularly, this invention relates to an imagefrequency suppression system for permitting substantially uniform imagefrequency suppression in two or more frequency hands.

This application is a continuation in part of the copending applicationof Harold A. Wheeler for Selective circuits, Serial No. 590,173, filedFebruary 1, 1932.

The invention is particularly applicable to superheterodyne radioreceivers. It is well known that the superheterodyne receiver isparticularly sensitive to voltages of two frequencies when set toreceive any given frequency. The

the amount of the intermediate frequency to which theintermediate-frequency amplifier is permanently tuned.

It is common practice to employ a signal-selecting system to amplify thedesired frequency of the two frequencies to which the superheterodynereceiverresponds, the desired frequency herein being called the signalfrequency, and which signal-selecting system attenuates the undesiredcarrier frequency or disturbance occurring at the other frequency towhich the recelver is particularly responsive, herein called the imagefrequency.

The attenuation of the image frequency relative to the signal frequencymay be increased by simultaneously tuning several selective circuits tothe signal frequency. However, if the station transmitting at the imagefrequency comes in rather strong, it is difficult to differentiatebetween the signal and image frequencies. Furthermore, in a receiverwhich is adapted to operate over several frequency bands, the necessityfor altering'the constants of several radiofrequency tuned circuitsincreasesthe disadvantage of this type of image frequency attenuation.

Itv is the object of the present invention to overcome the above-notedobjections to the superheterodyne radio receiver and provide a cirtwofrequencies differ from each other by twice.

frequency suppression throughout each of said bands.

These and further objects of this invention will become apparent fromthe following specification and claims taken in connection with theappended drawing.

For accomplishing the objects of this invention, there is providedbetween the antenna circuit and the input terminals of the receiver, apair of transfer means for each of the several '65 ranges through whichthe receiver is adapted to be tuned. The first transfer means includes atunable circuit which selectively transfers a voltage of the signalfrequency and, incidentally, also transfers a voltage of the undesiredimage v frequency, though the latter is considerably attenuated. Thesecond transfer means non-selectively transfers voltages of all imagefrequencies corresponding to the signal frequencies within the bandthrough which the receiver is adjusted to operate. The intensity of thisimage frequency voltage from the second transfer means is proportionedto be the same as that of the image frequency voltage incidentallytransferred through the selective transfer means. 30 The image frequencyvoltages from the two transfer meansare applied in opposite polaritiesto the input terminals of the first tube of the receiver, and thereforeimage frequency response is balanced out in said tube. Although the sec-35 0nd transfer means, incidentally, also transfers a voltage at thesignal frequency, the first transfer means effects a much greatertransfer than the second at this frequency, so that the second means hasbut little effect upon the transfer of the desired signal frequencies.Also, the signal frequency voltages transferred by these two means arenot in opposition at the desired signal frequency.

The circuits are so designed and proportioned that the frequency ofcomplete suppression varies automatically with the resonant frequency ofthe tunable circuit.

The theory underlying this part ofthe invention has'been described incomplete detail in the mo copending application above referred to andwill not therefore be further described in this application.

Provision is made for altering the tuninsband of the tunable transfermeans so that the variable 5 tenna circuit to make the antenna circuitbroadly resonant in the selected signal frequency band. Thenon-selective image transfer between the antenna and the input terminalsis thus altered by the second provision to give optimum or nearlyoptimum suppression of image interference with signals in the bandselected'by the first provision.

Optimum suppression is that obtained when substantially equal andopposite image voltages are transferred by the two respective transfermeans.

This system results in a great attenuation of the' image frequency overand above thatrdue to the selectivity of the tunable circuit in each ofthe several bands through which the receiver is designed to operate withimage suppression. It is an inherent feature of this invention that theincidental non-selective transfer of signal frequency voltage generallycauses substantially no change in the sensitivity ofthe receiver to thedesired signal.

Attention is now invited to the accompanyingdrawing in which the singlefigure shows the antenna circuit and tunable circuit associated with asuperheterodyne receiver.

In this figure, the operation of which will first be described with theswitches 25 and 26 open, and the system operating in the lower of itstwo frequency bands, the antenna circuit includes the antenna 10, thetrap circuit 12, series condenser 13, series coils 14 and 15, fixedcoupling condenser 16, and the ground terminal 11. The trap circuit 12is tuned to the intermediate frequency to prevent the direct receptionof currents of that frequency. The resistor28 is connected across partof the antenna circuit. Constants of the antenna circuit are so chosenthat the'entire antenna circuit is broadly resonant at about the middleof the lower frequency band,

the broadness of resonance being determined largely by the resistor 28.

The variable tuning circuit 17 comprises the coil 18 and the variablecondenser 19, and in addition, the coil 15 and the condenser 16 whichare common to the antenna circuit. The fixed condenser 16 has a maximumreactance which is about one-tenth to one-thirtieth the minimumreactance of either of elements 18 or 19 of the parallel tunable circuit17, in each frequency band.

The coupling between the antenna circuit and the tunable circuit 17 iscomposed of the coil 15 and the condenser 16. These twocouplingcomponents are in opposition by virtue of their opposite kindsof reactance. At all image frequencies, corresponding to the tuningrange, the coil 15 has a reactance less than that of the condenser 16.The effect of the coil 15 is material at the higher frequencies; but issmall or negligible at the lower frequencies.

The parallel tunable circuit 17, coupling coil 24, coil 22, and biasingresistor and condenser 21 comprise the input circuit of the receiver andare connected in series between the input terminals A-A of the receiver,namely, the grid and cathode of the modulator or first detector tube 20.The parallel tunable circuit constitutes the first or selective transfermeans between the antenna and input circuits.

The coil 22 is inductively coupled to a voltage source 23, the frequencyof which is varied, with the tuning of the circuit 17 by the condenser19, but which remains at a fixed frequency difference relative theretoequal to the intermediate frecal linkage indicated by the dotted lines.

quency to which the intermediate-frequency amplifiers of the receiverare tuned. The'frequency .of the source 23 is preferably higher than thecoupling constitutes the non-selective transfer means. 1

By this arrangement, two transfer means are provided between the antennaand the modulator tube. The first means includes the parallel tunablecircuit 17 which is adapted to selectively transfer a voltage of thedesired signal frequency and which attenuates voltages of the imagefrequency. The second transfer means comprises the mutual inductancebetween coils 14 and 24, which means non-selectively transfers a voltagefrom the antenna circuit to the input terminals of the modulator 20. Thecharacteristics of these two transfer means are so chosen that thevoltages thus impressed between the cathode and the tunable circuit 17are opposed to those transferred through the tunable circuit 17 and areequal at the image frequency.

The relative proportions of the two components of the coupling betweenthe antenna and the parallel tunable circuit 17 are such that thealignment between the image frequency and the frequency which issuppressed to the greatest degree by means of the two transfer meansabove described is exact or nearly exact throughout the entire tuningrange, thus giving optimum or nearly optimum image suppression.

The circuit and features above described are, as has been mentionedabove, those which are involved when the receiver is adjusted to operatein the lower of two frequency ranges. When it is desired to operate thesame circuits over the higher of two frequency ranges, the switches 25and 26 are simultaneously closed by a mechani- Closure of switch 25short-circuits a portion of the coil 18, to permit the condenser 19 totune the parallel tunable circuit 17 over the high frequency band, andclosure of switch 26 connects the auxiliary image frequency coil 27substantially in parallel with the coil 14. The inductance of coil 14 issubstantially greater than that of coil 27, and the larger portion ofthe current will fiowthrough the latter coil and coils 14 and 15 willhave relatively little effect on the operation. The total mutualinductance, however, is chosen to give substantially optimum imagesuppression with the switches 25 and 26 in the closed position, forpermitting operation in the highfrequency band. The image frequencysuppression is obtained in this band in substantially the same manner asdescribed above for the low frequency band. However, the inductance ofthe coil 27 is so chosen that when it is included in the antenna circuitas when operating within the high-frequency band, the entire circuitwill be broadly tuned at about the middle of the highfrequency band.

The output of the modulator 20 is connected to the input of the radioreceiver 29 which is, as indicated, responsive to currents of theintermediate frequency. This portion of the receiver, which is wellknown in the art and therefore need not be further described, amplifiesand reproggces the signals supplied to it by the modulator The junctionof the coil 15 and condenser 16 of grid-bias voltage in the receiver 29.This source is preferably varied automatically in an automatic volumecontrol (AVG) system, the spe-' ciflc details "of which constitute nopart of the present invention.

With the circuit which has been'described, it is possible to obtainimage frequency suppression throughout either of two frequency rangesand to alter simultaneously the resonance characteristics of theantennaand tunable circuits as desired by the simple manipulation of twoswitches connected for uni-control operation.

The-frequency controlling element of the generator circuit 23 may beconnected to be operated in a. uni-c ontrol manner with the variablecondenser 19 of the tunable circuit 1'7.

Appropriate current may be supplied to heat the cathode of the modulatortube 20, preferably by the usual power supply means included in thereceiver 29'.

Th circuit shown and described can be designed to operate over any twofrequency bands and the bands need not necessarily be contiguous.However, in a specific arrangement constructed in accordance with thisinvention, the lower frequency band comprised 550 kilocycles to 1400kilocycles and the next adjacent higher frequency band comprised 1400kilocycles to 3550 kilocycles. An intermediate frequencyof, 450kilocycles was utilized. The image frequency was 900 kilocycles higherthan the signal frequency.

Furthermore, the principles involved in this invention may be utilizedin a receiver designed to operate over three or more frequency bands.

The expression nearly optimum as used in the claims refers to circuitrelations which are optimum or which approach optimum nearly enough tosecure a substantial advantage in the suppression of image interference.

There is claimed: a

- 1. In combination with a superheterodyne receiver operative over ahigh-frequency band and a low-frequency band, the arrangement forcoupling anantenna to said receiver and suppressing image interferenceover both of said bands; which comprises a parallel tuned circuit and acoupling coil connected in series between theinput terminals of saidreceiver, an antenna ceiver operative over a high-frequency band and alow-frequency band, the arrangement for coupling an antenna to saidreceiver and suppressing image interference over both of said bandswhich comprises a parallel tuned circuit and a coupling coil connectedin series between the input terminals of said receiver, an antennacircuit including a series coil, a condenser common to and coupling saidantenna and parallel tuned circuits, image-suppressing mutual inductancebetween said coils, and switching means for simultaneously adjustingsaid tuned circuit to operate overeither of said bands, adjusting saidseries coil to resonate the antenna circuit within the selected band,and selecting. a. predetermined value of said mutual inductance whichgives nearly optimum image suppression in the same band.

3. In combination with a superheterodyne receiver operative over ahigh-frequency band and a low-frequency band, the arrangement forcoupling an antenna to said receiver andsuppressing image interferenceover both of said bands,

which comprises a parallel tuned circuit and a coupling coil connectedin series between the input terminals of said receiver, an antennacirwhich gives nearly optimum image suppression in the same band.

4. In combination with a superheterodyne receiver operative over ahigh-frequency band and a low-frequency band, the arrangement forcoupling an antenna to said receiver and'suppressing image interferenceover both of said bands,

which comprises a parallel tuned circuit and a coupling coil connectedin series between the input terminals of said receiver, an antennacircuit including a series coil, a condenser common to and coupling saidantenna and parallel tuned circuits, image-suppressing mutual inductancebetween said coils, and switching means for adjusting said tuned circuitto operate over either of said bands, and for selecting a predeterminedvalue of said mutual inductance which gives nearly optimum imagesuppression in the same band.

5. In combination with a superheterodyne receiver operative over ahigh-frequency band and a low-frequency band, the arrangement forcoupling an antenna to'said receiver and suppressing image frequencyinterference over both of said bands, which comprises an input circuitincluding a parallel tunable circuit and a coupling coil connected inseries between the input terminals of said receiver, an antenna circuitincluding a series coil and, alternatively, a second coil in shunttherewith, a condenser common to and coupling saidantenna and paralleltunable circuits, the coils of said antenna circuit being inductivelycoupled'to the coupling coil in the input circuit, the coupling thusprovided between the antennaand the input circuits being proportioned tobalance out image frequency voltages transmitted to said input circuitthrough the coupling between said antenna-and parallel tunable circuits,a switch for altering one of the constants of said parallel tunablecircuit to permit tuning thereof over each of said frequency bands, anda second switch operatively connected to said first-mentioned switch foralternatively connecting the second series'coil of said antenna circuitin parallel with the first coil thereof and thereby simultaneouslyaltering the constants of said antenna circuit to cause it to be broadlyresonant within the selected band and selecting the proper inductivecoupling between said antenna.

and input circuits to give nearly optimum image suppression in the sameband.

6. In combination with a superheterodyne receiver operative over ahigh-frequency band and its a low-frequency band, the arrangement forcouimage frequency interference over both of said bands, which comprisesan input circuit including a parallel tunable circuit and a couplingcoil connected in series between the input terminals of said receiver,an antenna circuit including a series coil, and a condenser common toand coupling said antenna and parallel tunable circuits, the series coilof said antenna circuit being inductively coupled to the coupling coilin the input circuit, the coupling thus provided being proportioned tobalance out image frequencyvoltages transmitted to said input circuitthrough the common coupling condenser, a second series coil foralternative connection in parallel with the first series coil of saidantenna circuit, a switch for altering one of the constants of saidparallel tunable circuit to permit tuning thereof over each of thefrequency bands, and a second switch operatively connected to saidfirst-mentioned switch for alternatively connecting said second seriescoil in parallel with the first series coilof said antenna circuit andfor simultaneously altering the constants of said antenna circuit tocause it to be broadly resonant within the selected band and selectingthe proper inductivev coupling between said antenna and input circuitsto give nearly optimum image suppression in the same band.

7. In combination with a superheterodyne receiver operative over ahigh-frequency band and a low-frequency band, the arrangement forcoupling an antenna to said receiver and suppressing image frequencyinterference over both of said bands, which comprises an antenna circuitand an input circuit, a tunable transfer means coupling said circuits,an untuned transfer means also coupling said circuits, thecharacteristics of said untuned transfer means being so chosen as totransfer voltages at the image frequency which are-equal and opposite tothose incidentally transferred through said tunable transfer means, aswitch means for altering a characteristic of said tunable transfermeans to permit tuning over either of said frequency bands, and a secondswitch means operatively connected to said first switch means forsimultaneously altering the characteristics of said untuned transfermeans to give nearly optimum'image suppression in the selected band andto tune the antenna circuit to be broadly resonant within the same band.

8. In combination with a superheterodyne receiver operative selectivelyover a high-frequency band and a low-frequency band, the arrangement forcoupling an antenna to said receiver and suppressing image frequencyinterference over both of said bands, which comprises an input circuithaving a parallel tunable circuit and a coupling coil connected inseries between the input terminals of said receiver, an antenna circuitinclud'ng a series coil, and a condenser common to and coupling saidantenna circuit and said pling an antenna to said receiver andsuppressing parallel tunable circuit, the series coil of said antennacircuit and the coupling coil of said input circuit being inductivelycoupled thus providing two coupling paths between. said antenna andinput circuits whereby the image frequency currents transferred throughsaid inductive coupling will balance out those transferred through saidparallel tunable circuit, and a switch connected to said circuits forsimultaneously altering said inductive coupling and the constants ofsaid antenna circuit to cause it to be broadly resonant within theselected'band and to give nearly optimum image suppression in the sameband.

9. In combination with a superheterodyne receiver selectively operativeover a plurality of frequency bands, the arrangement for coupling anantenna to. said receiver and suppressing image frequency interferenceover each of said bands, which comprises an antenna circuit and an inputcircuit, a tunable transfer means coupling said circuits, an untunedtransfer means also coupling said circuits, the characteristics of theuntuned transfer means being so chosen as to transfer voltages attheimage frequency which are equal and opposite to those incidentallytransferred through said tunable transfer means, and a-switch meansconnected to said circuits for simultaneously altering .thecharacteristics of said untuned transfer means to give nearly optimumimage suppression in any desired frequency band and to tune the antenna,circuit to be broadly resonant within the same band.

10. In combination with a superheterodyne receiver selectively operativeover a plurality of frequency bands, the arrangement for coupling anantenna to said receiver and suppressing image frequency over any ofsaid bands, which comprises an input circuit having connected in seriesbetween the input terminals of said receiver a parallel tunable circuitand a coupling coil, an antenna'circuit including a series coil, and acondenser common to and coupling said antenna and parallel tunablecircuits, the coil of said antenna circuit being inductively coupled tothe series coil in the input circuit, the coupling thus provided beingproportioned to balance out image frequency voltages transmitted to saidinput circuit through the common coupling condenser, a second seriescoil for alternative connection in parallel with the first series coilof said antenna c'rcuit, and a switch connected to said circuits forconnecting said second series coil in parallel with the first seriescoil to said antenna circuit and thereby simultaneously altering theconstants of said antenna circuit to cause it to be broadly resonantwithin the selected band and simultaneously selecting the properinductive coupling between said antenna and input 'circuits to givenearly optimum image suppression in the same band.

